replace DifferentialEquations dependency from tests

[j-fu]

Hi,
besides using SciMLBase instead of DifferentialEquations.jl in the package code, the compilation load for testing + docs can be reduced as well. It would be sufficient to use OrdinaryDiffEq instead.
But recently, this has been split further, e.g. there is now OrdinaryDiffEqRosenbrock etc., see
https://docs.sciml.ai/OrdinaryDiffEq/stable/usage/
So it is now sufficient to use e.g. OrdinaryDiffEqRosenbrock. Probably due to https://github.com/SciML/OrdinaryDiffEq.jl/blob/6ec790ae0d7c4e45c29bfd3220203040451d47a5/lib/OrdinaryDiffEqRosenbrock/src/OrdinaryDiffEqRosenbrock.jl#L37 , this makes solve available as well.

[chmerdon]

solved by PR#27